There are many geographic locations around the world where it is highly desirable to hydrolyze hemicellulose and cellulose in biomass, particularly waste cellulose containing products. For example it is highly desirable to be able to hydrolyze the hemicellulose and cellulose in agricultural wastes (such as bagasse), waste paper, and cellulose-containing municipal solid wastes.
In general, biomass contains cellulose, hemicellulose, lignin and ash. When the biomass is treated with hot dilute mineral acid the hemicellulose and cellulose will be hydrolyzed. The hemicellulose is typically converted to primary five carbon sugars, while the cellulose is converted to primary six carbon sugars. The soluble sugars can be separated from the lignin solids by conventional means, leaving a sugar solution. The sugar solution can be fermented into ethanol, which--after purification--has many end uses, including as a fuel additive. Exemplary procedures and apparatus for alcohol production from biomass are disclosed in U.S. Pat. Nos. 4,436,586 and 4,612,286.
One particularly desirable step in the production of ethanol from biomass is pre-hydrolysis. Pre-hydrolysis comprises the hydrolyzing of hemicellulose to sugars (while the cellulose is not hydrolyzed). In addition to hydrolyzing the hemicellulose, pre-hydrolysis swells the cellulose, which enhances the effectiveness of subsequent hydrolysis of the cellulose to simple sugars.
According to the present invention a method and apparatus are provided for acid pre-hydrolysis of biomass. The invention provides a simple yet effective manner for practicing pre-hydrolysis. The method and apparatus according to the invention may be used as a preliminary to a wide variety of other techniques for acting on the biomass being treated, or the hydrolysate produced, such as hydrolyzing of the cellulose, production of paper pulp, production of alcohol and furfural, and the like.
According to one aspect of the present invention a method of acid pre-hydrolysis of biomass, effecting hydrolysis of hemicellulose to sugars, is provided. The invention comprises the steps of automatically, continuously, and sequentially: (a) Mixing biomass containing hemicellulose with a mineral acid solution of sufficient concentration to eventually hydrolyze the hemicellulose therein, and to form a slurry having a consistency that insures proper wetting of the biomass with mineral acid. (b) Dewatering the slurry so as to minimize the amount of steam required to heat the biomass to reaction temperature and to maintain a desired sugar solution concentration after prehydrolysis. (c) Heating the dewatered slurry to reaction temperature, at superatmospheric pressure by direct contact with steam. And, (d) retaining the biomass in the dewatered slurry at reaction temperature and pressure conditions for a time sufficient to hydrolyze the hemicellulose of the biomass.
Step (a) can be practiced using biomass such as agricultural wastes (bagasse), waste paper, and municipal solid cellulose waste. Step (a) is also practiced to form a slurry having a consistency of about 8 to 12%, while step (b) is practiced to dewater the slurry to a consistency of about 35 to 50%. During the practice of step (b) a pressate is produced, and the invention also comprises the steps: (e) Adding mineral acid to the pressate to form a mixture of pressate and acid. (f) Heating the mixture of pressate and acid. And (g), using the heated mixture as the acid solution source in step (a). Step (f) is practiced by (g) washing the biomass from step (d) to produce a sugar solution having a temperature well above 100.degree. F., and (h) passing the sugar solution into indirect, heat exchange, contact with the mixture of pressate and acid. Also there preferably is the further step between steps (b) and (c) of fluffing the dewatered slurry. The amount of acid solution added in step (a) is also typically automatically controlled in response to the mass flow rate of biomass used in step (a).
In the typical practice of the method of the invention, steps (c) and (d) are practiced in a pressurized vertical reactor, and step (c) takes place in a vapor phase at the top of the reactor, and comprises the further steps of diluting and cold blowing the biomass from the bottom of the reactor. Steps (c) and (d) are practiced at a pressure which typically is about 30 to 50 psig (e.g. about 40 psig), at a temperature of about 250.degree.-350.degree. F. (e.g. about 320.degree. F.), and for a time of about 20 to 40 minutes (e.g. about 30 minutes). Step (b) may be practiced in a dewatering press, and there preferably is the further step of preventing vapors from the top of the reactor from passing back to the dewatering press. The consistency of the biomass slurry at the top of the reactor is typically about 34%, and 24% at the bottom, and the diluting step is practiced to dilute the biomass to a consistency of about 8 to 12%. Cold blowing takes place at a temperature of about 180.degree. F.
According to another aspect of the present invention a method of acid pre-hydrolysis of biomass, effecting hydrolysis of hemicellulose to sugars, comprises the automatic, continuous, and sequential practice of the following steps: (a) Mixing biomass containing hemicellulose with a mineral acid solution of about 0.5-3.0% (typically about 1%) to form a slurry having a consistency of about 8-12%. (b) Dewatering the slurry to a consistency of about 35-50%. (c) Heating the dewatered slurry to a temperature of about 250.degree.-350.degree. F., at a pressure of about 30-50 psig, by direct contact with steam. And, (d) retaining the biomass in the dewatered slurry at reaction temperature and pressure conditions for a time sufficient to hydrolyze the hemicellulose of the biomass.
According to yet another aspect of the present invention, a pre-hydrolysis reactor system is provided. The reactor system comprises the following elements: A mixer having a first inlet for substantially dry biomass, and a second inlet for acid solution, and an outlet for slurried biomass. An impregnation press having an inlet connected to the outlet from the mixer, a dewatered biomass outlet, and a pressate outlet. A vertical reactor having a top and a bottom, and first and second inlets at the top thereof for biomass slurry and steam, respectively, arranged to provide a vapor phase at the top of the reactor. High density feed means connected between the impregnation press biomass outlet and the reactor first inlet for feeding high density biomass into the vertical reactor first inlet. And, a cold blow pre-hydrolyzed biomass outlet from the bottom of the reactor. A fluffer is preferably disposed in the top of the reactor between the first and second inlets, the second inlet being vertically above the first inlet.
The reactor system also may comprise the following further additional elements: A pressate tank having an inlet and an outlet. A pressate line extending from the impregnation press pressate outlet to the pressate tank inlet. An indirect heat exchanger. A pump. A transport line interconnecting the pressate tank outlet, pump and heat exchanger so that pressate is pumped from the tank through the heat exchanger. An acid addition line connected to the transport line between the pressate tank and the pump. And, a discharge line from the heat exchanger connected to the second inlet to the mixer. A weigh conveyor may be operatively connected to the mixer first inlet, and a flow control valve disposed in the discharge line from the heat exchanger. A controller is provided for controlling the flow control valve in response to the weigh conveyor.
Typically the mixer is disposed vertically above the impregnation press, and the inlet to the impregnation press and the outlet from the mixer are connected by a feed chute. The impregnation press is vertically above the high pressure feed means (which may comprise a high density feed pump), and the impregnation press biomass outlet is connected to the high pressure feed means inlet by a chute. The high pressure feed means also comprises means for preventing passage of vapors from the reactor back to the impregnation press.
It is the primary object of the present invention to provide a simple yet effective method and apparatus for acid pre-hydrolysis of biomass to hydrolyze hemicellulose to sugars. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.